Soil microbes can play a role in plant diversity maintenance. Plant species often cultivate distinct soil microbial communities, which then influence the performance of the cultivating plant and other species in the system. These plant-soil feedbacks (PSFs) can stabilize plant coexistence by generating negative frequency dependence or destabilize it through positive frequency dependence. Recent research has shown that PSFs can also give one species a frequency-independent average fitness advantage over the other, and that inferring the net effect of soil microbes on plant coexistence requires comparing the magnitude of this microbially mediated fitness difference to the stabilization they generate. However, empirical PSF studies have largely overlooked the effect of fitness difference. We conducted a two-phase greenhouse experiment to quantify both the microbially mediated stabilization and fitness difference in 15 California annual species pairs and used both metrics to predict coexistence outcomes. We also parameterized recently-developed metrics of community-wide stabilization in multispecies plant communities to evaluate the role of PSFs on plant dynamics in diverse systems.
Results/Conclusions
Plant species differed in their responses to soil microbes cultivated by conspecifics and heterospecifics, setting up the basis for strong plant-soil feedbacks. The average effect of these feedbacks was to stabilize all but one species pair. However, soil microbes also generated strong fitness differences that negated their stabilizing effects in 11 out of 15 pairs. Therefore, assuming that plant species are otherwise equivalent competitors, the net effect of PSFs is to drive exclusion among the species in our study, a qualitatively different result from considering stabilization alone. Parameterizing metrics of PSF-mediated stabilization in multispecies communities shows that it can be difficult to infer community-wide coexistence from pairwise analyses alone. Our multispecies analysis showed that PSFs can destabilize multispecies communities even when they promote coexistence among each constituent species pair in the system. On the other hand, we found that PSFs may stabilize a multispecies community without stabilizing any single constituent pairwise interaction. Our work highlights the value of linking experimental results with recent developments in PSF theory to advance our understanding of how soil microbes influence plant diversity.